1. Field of the Invention
The present invention relates to method and apparatus for extracting a pitch period (or a reciprocal thereof, that is, pitch frequency) in speech analysis, and more particularly to a method and apparatus for extracting speech pitch suitable for real time analysis.
Description of the Prior Art
Significance of pitch period extraction which is a main portion of sound source information in extracting information in a speech compression system or speech analysis-synthesis has been experimentarily recognized since the invention of the vocoder in 1939 (The Vocoder by H. Dudley, Bell Labs. Record, 17, 122-126, 1939). A number of investigations and experiments have been reported on the pitch period extraction method since Dudley's invention. A representative one of them is reported by "Speech Analysis" (IEEE Press, John Wiley Sons Inc. 1978), Part III, Estimation of Excitation Parameters, A Pitch and Voicing Estimation, which is one of IEEE Press Selected Reprint Series edited by R. W. Schafer and J. D. Markel. However, a decisive pitch extraction method has not been established yet and investigation and experiment reports have been continuously contributed to domestic and foreign associations.
As a so-called linear prediction analysis and synthesis method has been recently researched and developed and a speech synthesis LSI has been realized, the need for the pitch extraction method has further increased and the establishment of reliable pitch extraction method in the real time analysis is a significant point to improve the tone quality of transmitted or synthesized sound and the significance thereof is increasing to an even greater extent.
Most of prior art approaches to the improvement of the pitch extraction method are mainly directed to off-line analysis and they are not always suited to real time analysis.
In pitch extraction, a 1/2, 1/3, double or triple period is often detected. The difficulty in pitch extraction resides in a specific manner of determination thereof and a specific manner of maintaining the continuity of the extracted result. A beginning of a word or an ending of a word generally has a small amplitude and the pitch period thereof is not always definite. Nevertheless, in the real time analysis a process has to be started from an ambiguous state.
However the pitch extraction method is improved, it is difficult to completely resolve the above problem and some countermeasurement is needed in processing the extracted result.
In the real time analysis, it is not permitted to start the process after the pitch has been positively extracted or the analysis has been completed. This adds a further difficulty.
The prior art approaches to the above problems are not always sufficient. Most approaches have disadvantages in that the process is started after data and information have been stored.